In the past, in measuring instruments such as a laser radar (ridar), for example, from the viewpoint of eye-safe, near infrared light with a wavelength of 1550 nm is projected from a light source and the light is received by a light-receiving element to measure the object. Currently, although there are various options regarding the light source, options for the light-receiving element are limited, and there are many problems.
As a conventional light-receiving element having a light-receiving sensitivity to these near infrared lights, from the viewpoint of low noise and fast response speed, for example, a compound semiconductor such as indium gallium arsenide (InGaAs) is often used. However, the method using indium gallium arsenide (InGaAs) has a problem that productivity is very poor and high manufacturing cost is required. Therefore, there is a need for a new light-receiving element which is high in productivity and may suppress the manufacturing cost.
Incidentally, a light-receiving element using germanium (Ge) as an absorbing layer is known as a light-receiving element having a light-receiving sensitivity in the near infrared region around a wavelength of 1550 nm without using indium gallium arsenide (InGaAs).
As such a light-receiving element, by using germanium (Ge) or silicon (Si)-germanium (Ge) as an intrinsic semiconductor, an optical element that absorbs light having a wavelength in the near-infrared region and is suitably usable for applications such as optical communication is disclosed (Patent document 1). Patent document 1 discloses an avalanche photodiode (APD) having a p-doped region, an intrinsic region and an n-doped region, and at least one of a p-doped region and an n-doped region is arranged in an array.
Further, as another example of the light-receiving element, a configuration of an avalanche photodiode (APD) having germanium (Ge) as an absorption layer and silicon (Si) as an amplification layer by growing germanium (Ge) on a silicon (Si) layer has been disclosed (Non-patent document 1). According to the light-receiving element of Non-patent document 1, although it is known that germanium (Ge) has a lot of noise, but by using silicon (Si) as an amplifying layer, it is possible to produce a sensor having a reduced noise and having sensitivity to the wavelength in the near infrared region as described above.
Since these optical elements are assumed to be used for optical communication applications, they are configured to have low power consumption and a high response speed. For this reason, usually, they have a configuration of using an absorption layer formed in a waveguide-shape to propagate and absorb light (refer to FIG. 10). Since the interaction length (L2 in FIG. 10) for absorbing light may be made long even if the thickness of the waveguide-shape absorbing layer is made thin, it is possible to suppress noise due to dark current, and the speed may be increased. In addition, since the applied voltage may be suppressed, power consumption may also be suppressed.
However, these optical elements are supposed to be used for applications for optical communication, and it is difficult to use them for receiving light from free space. In particular, germanium (Ge) has a very large refractive index of about 4, and light from free space often has a large incident angle, so that the ratio of reflection at the absorption layer surface is also large. Accordingly, light is not efficiently absorbed in the absorbing layer. In addition, as described above, the light-receiving element used for optical communication uses a thin absorption layer, so that when used as a light-receiving element for receiving light from free space, the interaction length (L1 in FIG. 3) for absorbing light becomes short, and there arises a problem that light is not sufficiently absorbed in the absorption layer. Also, since the absorption layer made of germanium (Ge) has a very large noise, merely increasing the layer thickness of the absorption layer slows down the response speed and the noise becomes very large. Consequently, it is difficult to use these elements for receiving light from free space.